Many pH-responsive thickeners are known in the art and are used to thicken water-based compositions. These thickeners are generally based upon the incorporation of a hydrophobic surfactant monomer into a hydrophilic, polymeric backbone. The traditional backbone composition for these thickeners primarily included monomeric acid, such as acrylic or methacrylic acid, and an alkyl acrylate or methacrylate, such as ethyl acrylate. The hydrophobic surfactant component is primarily derived from a polyethoxylated alkyl group. Several patents disclose pH-responsive thickeners and the various linkages connecting the hydrophobic surfactant component to the polymer backbone.
U.S. Pat. No. 4,384,096 discloses liquid emulsion polymers useful as pH-responsive thickeners containing an acrylate or methacrylate linkage. U.S. Pat. No. 4,569,965 discloses crotonate-containing polymeric thickeners. U.S. Pat. No. 4,464,524 discloses polymer thickeners containing maleate linkages. U.S. Pat. No. 4,663,385 discloses copolymers of alkyl poly(oxyalkylene)itaconic diesters. U.S. Pat. No. 4,616,074 discloses acrylic-methylene succinic ester emulsion copolymers for thickening aqueous systems. U.S. Pat. No. 4,338,239 discloses thickener copolymers having allyl glycidyl ether linkages. U.S. Pat. No. 4,514,552 discloses alkali soluble latex thickeners containing urethane linkages. U.S. Pat. No. 4,600,761 discloses thickener copolymers containing isocyanato ethyl methacrylate linkages. European patent publication No. 0216479 discloses polymeric thickeners containing allyl ether linkages.
The polymeric thickeners of the prior art had several disadvantages which adversely affected their performance in paint applications. The disadvantages of the prior art thickeners were loss of viscosity upon heat-aging and colorant addition, decreased scrub resistance, and blistering over chalky substrates. The present invention overcomes nearly all these disadvantages of the prior art thickeners by providing multi-stage polymer particles wherein the ionically-soluble thickener copolymer (preferably base-soluble) has been physically or chemically attached (either directly or indirectly) to another (preferably base-insoluble) polymer stage. When the polymer particles of this invention are incorporated into water-containing compositions and neutralized with a base or acid, a portion of the ionically-soluble polymer stage remains attached to the remainder of the polymer particle.
Mixtures or blends of alkali-soluble polymers with alkali-insoluble polymers are known in the art, such as described in U.S. Pat. No. 3,037,952. Additionally, European Patent Publication No. 0207854 discloses a coating composition containing core-shell polymer particles containing (A) 95-99% by weight of at least one C.sub.1 -C.sub.8 alkyl acrylate or methacrylate and (B) 1-5% by weight of at least one water soluble monomer. However, none of these references teach multi-stage polymer particles which are useful as thickeners comprising a hydrophobically-modified, ionically-soluble polymer stage.